<header>
  <nav>
    <ul class="header-inner">
      <li><a href="#section1">section1</a></li>
      <li><a href="#section2">section2</a></li>
      <li><a href="#section3">section3</a></li>
      <li><a href="#section4">section4</a></li>
      <li><a href="#section5">section5</a></li>
    </ul>
  </nav>
</header>
<main>
  <h2 id="section1" class="section">section1</h2>
  <h2 id="section2" class="section">section2</h2>
  <h2 id="section3" class="section">section3</h2>
  <h2 id="section4" class="section">section4</h2>
  <h2 id="section5" class="section">section5</h2>
  <p class="back-btn"><a href="#">Topに戻る</a></p>
</main>

* {
  box-sizing: border-box;
  list-style: none;
  margin: 0;
  padding: 0;
  text-decoration: none;
}

/* ゆっくりに遷移する */
html {
  scroll-behavior: smooth;
}

.header-inner {
  align-items: center;
  background-color: #111934;
  display: flex;
  height: 80px;
  justify-content: space-around;
  position: fixed;
  top: 0;
  width: 100%;
}

.header-inner li {
  line-height: 80px;
  text-align: center;
  transition: .5s;
  width: 100%;
}

.header-inner li:hover {
  background-color: #717171;
  color: #111934;
}

.header-inner li:not(:nth-of-type(5)) {
  border-right: 1px solid #fff;
}

.header-inner li a {
  color: #e8e8e8;
  display: block;
}

@media screen and (max-width: 480px) {
  .header-inner li a {
    font-size: 12px;
  }
}

.section {
  background-color: #b5b5b5;
  display: flex;
  font-size: 40px;
  height: 120vh;
  justify-content: center;
  width: 100%;
  scroll-margin-top: 80px;
}

.section:nth-of-type(1) {
  padding-top: 80px;
}

.section:nth-of-type(even) {
  background-color: #302f2f;
  color: #e8e8e8;
}

.back-btn {
  background-color: #111934;
  border-radius: 16px;
  bottom: 20px;
  height: 50px;
  line-height: 50px;
  position: fixed;
  right: 20px;
  text-align: center;
  transform: translateX(-50%);
  width: 120px;
}

.back-btn a {
  border-radius: 16px;
  color: #e8e8e8;
  cursor: pointer;
  display: block;
  height: 100%;
  transition: .3s;
  width: 100%;
}

.back-btn a:hover {
  background-color: #717171;
  color: #111934;
}